A "class not found" error is a caching thing with your browser. Clearing the cache should work.
The correct way to do that is clear the cache, close all browser windows you have open, and then reload the chat. You might run into the same thing then. Hitting refresh (maybe a couple of times) should help then.
We've had this problem when Raidersoft upgraded a while ago. I've had it myself, and it was quite a sticky problem. I solved it back then by upgrading my java thingie from Sun. That definitely reinitiates the cache, hehe.
And RZ500, seems like you have a unique problem there. I think I need more info, but I wouldn't know what kind of info. Things work, or things dont work. When things work sometimes it gets really difficult.